Recently, liquid crystal displays are extensively used as display devices of, e.g., portable notebook personal computers. These liquid crystal display devices incorporate a cold-cathode fluoresent lamp as a so-called back light in order to illuminate a liquid crystal display panel from the back. Turning on this cold-cathode fluoresent lamp requires a inverter capable of converting a low DC voltage of a battery or the like into a high AC voltage of 1,000 Vrms or more in an initial lighting state and about 500 Vrms in a steady lighting state. Conventionally, a winding transformer is used as a boosting transformer of this inverter. In recent years, however, a piezoelectric transformer which performs electric conversion via mechanical energy and thereby performs boosting is beginning to be used. This piezoelectric transformer has a generally unpreferable characteristic, i.e., largely changes its boosting ratio in accordance with the magnitude of an output load (load resistance). On the other hand, this dependence upon a load resistance is suited to the characteristics of an inverter power supply for a cold-cathode fluoresent lamp. Accordingly, a piezoelectric transformer has attracted attention as a small-sized, high-voltage power supply meeting the demands for a low profile and a high efficiency of a liquid crystal display device. A basic configuration of a control circuit for this piezoelectric transformer will be described below with reference to FIGS. 1 to 4.
FIG. 1 is a block diagram of a piezoelectric transformer control circuit as the first prior art.
In FIG. 1, reference numeral 101 denotes a piezoelectric transformer; 102, a load such as a cold-cathode fluoresent lamp connected to the output terminal of the piezoelectric transformer 101; 103, an oscillation circuit for oscillating an AC signal such as a rectangular wave; and 104, a driving circuit for driving the piezoelectric transformer 101 in accordance with the oscillation signal from the oscillation circuit 103.
It is generally known that a piezoelectric transformer largely changes the output voltage in the form of a hill in accordance with the frequency of an input AC voltage, the output voltage takes a maximum value when the piezoelectric transformer is driven by its resonance frequency, and the resonance frequency changes in accordance with the temperature or the magnitude (load resistance) of an output load. Accordingly, the general approach is to cause the oscillation circuit 103 to output an oscillation signal equal in frequency to the resonance frequency and drive the piezoelectric transformer 101 by the driving circuit 104 on the basis of this oscillation signal, thereby generating a high voltage at the output terminal of the piezoelectric transformer 101.
In the block diagram of FIG. 1, the driving circuit 104 can have an arrangement as shown in FIG. 2.
FIG. 2 is a block diagram of a piezoelectric transformer control circuit as the second prior art. In FIG. 2, a driving circuit 104 includes a p-type transistor (FET: Field-Effect Transistor) 104a and an n-type transistor (FET) 104b which are so connected as to form a half-bridge circuit. These two transistors (104a and 104b) alternately perform switching in accordance with the state of an output oscillation signal from an oscillation circuit 103. By this switching operation of the driving circuit 104, a driving voltage (AC voltage) whose amplitude is an input voltage Vi is applied to a piezoelectric transformer 101.
In the control circuits having the above configurations, it is necessary to change a lamp current (load current) flowing in a cold-cathode fluoresent lamp connected as the load 102 in order to control the brightness of the cold-cathode fluoresent lamp. To this end, the applied voltage (the output voltage from the piezoelectric transformer 101) to the cold-cathode fluoresent lamp must be adjusted. To adjust the applied voltage, it is necessary to regulate the oscillation signal from the oscillation circuit 103 as the basis of the applied voltage. The area in an ON period of this oscillation signal can be regarded as an energy amount supplied to the piezoelectric transformer 101. Accordingly, the output voltage from the piezoelectric transformer 101 can be changed by changing this energy amount. In conventional piezoelectric transformer control circuits, therefore, the brightness of a cold-cathode fluoresent lamp is controlled by methods as shown in FIGS. 3 and 4.
FIGS. 3 and 4 are timing charts for explaining conventional methods of controlling the brightness of a cold-cathode fluoresent lamp.
In the method shown in FIG. 3, the energy amount supplied from a driving circuit to a piezoelectric transformer is regulated by changing the amplitude of an oscillation signal, and thereby (the amplitude of) an output voltage is adjusted. In the method shown in FIG. 4, as disclosed in, e.g., Japanese Patent Laid-Open No. 5-64437 or 7-220888, a PWM (Pulse Width Modulation) circuit (not shown) is arranged in the control circuit shown in FIG. 1 or 2. In accordance with a signal from this PWM circuit, the duty ratio (Ton/(Ton+Toff)) of an oscillation signal from an oscillation circuit is changed to regulate the energy amount supplied from a driving circuit to a piezoelectric transformer, thereby adjusting (the amplitude of) an output voltage.
Unfortunately, when the brightness of a cold-cathode fluoresent lamp as a load is lowered by dropping the output voltage from a piezoelectric transformer by the above conventional methods, the lighting state becomes unstable if the output voltage from the piezoelectric transformer becomes lower than a voltage necessary to keep the cold-cathode fluoresent lamp discharging. The resultant flickering is of a problem to a human visual sense. Accordingly, adjusting the brightness by the above conventional methods has the problem that a dimming range within which a stable brightness is obtained is narrow.
A piezoelectric transformer control circuit as still another prior art capable of holding the brightness of a cold-cathode fluoresent lamp at a predetermined brightness will be described below with reference to FIGS. 5 to 10.
FIG. 5 is a block diagram of a piezoelectric transformer control circuit as the third prior art.
In FIG. 5, reference numeral 201 denotes a piezoelectric transformer; 202, a load such as a cold-cathode fluoresent lamp connected to the output terminal of the piezoelectric transformer 201; 203, a detecting resistor Rdet for detecting a current flowing in the load; 204, a rectifying circuit for converting an AC voltage generated in the detecting resistor 203 into a DC voltage; 205, an error amplifier for comparing a voltage Vri rectified by the rectifying circuit 204 with a reference voltage Vref and amplifying the difference as a comparison result; 206, a voltage-controlled oscillation circuit for outputting an oscillation signal in accordance with the output voltage from the error amplifier 205; and 207, a driving circuit for driving the piezoelectric transformer 201 in accordance with the oscillation signal from the voltage-controlled oscillation circuit 206. The operation of the control circuit with the above configuration will be described below with reference to FIGS. 6A and 6B.
FIG. 6A is a graph for explaining the relationship between the frequency and the output voltage of a piezoelectric transformer. FIG. 6B is a graph for explaining the relationship between the frequency of a piezoelectric transformer and the load current of a load connected to the piezoelectric transformer.
As shown in FIG. 6A, the piezoelectric transformer 201 has a hilly resonance frequency characteristic whose peak is the resonance frequency of the piezoelectric transformer 201. It is generally known that a current flowing in the load 202 due to the output voltage from the piezoelectric transformer 201 also has a similar hilly characteristic. In FIG. 6B, this load current is represented by a load current detection voltage Vri. Control using a right-side (falling) portion in this characteristic will be described below. When the power supply of this control circuit is turned on, the voltage-controlled oscillation circuit 206 starts oscillating at an initial frequency fa. Since no current flows in the load 202 at that time, the voltage generated in the detecting resistor 203 is zero. Accordingly, the error amplifier 205 outputs a negative voltage, as a result of comparison of the load current detection voltage Vri with the reference voltage Vref, to the voltage-controlled oscillation circuit 206. In accordance with this voltage, the voltage-controlled oscillation circuit 206 shifts the oscillation frequency of an oscillation signal to a lower frequency. Therefore, as the frequency is shifted to a lower frequency, the output voltage from the piezoelectric transformer 201 rises, and the load current (load current detection voltage Vri) also increases. When the load current (load current detection voltage Vri) and the reference voltage Vref become equal to each other, the frequency stabilizes (fb). Even if the resonance frequency changes due to a temperature change or a change with time, the frequency shifts accordingly to always hold the load current substantially constant.
In the control circuit shown in FIG. 5, therefore, frequency control is so performed that the load current detection voltage Vri becomes equal to the reference voltage Vref, and the load current is held at a predetermined value by this frequency control. When a cold-cathode fluoresent lamp is used as a load in this piezoelectric transformer control circuit and the control circuit is used as a lighting device for the cold-cathode fluoresent lamp, a function of holding the brightness of the cold-cathode fluoresent lamp at a predetermined brightness can be achieved since the brightness of the cold-cathode fluoresent lamp is proportional to a lamp current flowing in the cold-cathode fluoresent lamp.
As described above, the control circuit shown in FIG. 5 can hold the lighting state of a cold-cathode fluoresent lamp at a predetermined brightness. In practice, however, a function (dimming function) of changing the brightness is required in addition to the function of holding the lighting state at a predetermined brightness.
FIGS. 7 and 8 show other prior arts. FIG. 7 is a block diagram of a circuit having a function of holding the driving voltage to a piezoelectric transformer substantially constant. FIG. 8 is a block diagram of a circuit having a function of holding the output voltage from a piezoelectric transformer substantially constant. In these circuits, an error amplifier 205 compares a voltage detected by detecting resistors 210a and 210b and a rectifying circuit 204 with a reference voltage. In accordance with the comparison result, the duty ratio of an output rectangular-wave signal from a PWM oscillation circuit 211 which performs so-called PWM (Pulse Width Modulation) is controlled. When the duty ratio of this rectangular-wave signal is controlled, the amplitude of a sine wave contained as a fundamental wave in the rectangular wave can be changed. This enables control for holding the driving voltage or the output voltage at a predetermined value as in the case of the voltage-controlled oscillation circuit 206.
FIG. 9 shows still another prior art in which a lamp current is controlled by adjusting the duty ratio of an output rectangular-wave signal from a PWM oscillation circuit 211. In this control circuit, the lamp current is detected by using a resistor 203. Also, a control circuit shown in FIG. 10 uses the voltage-controlled oscillation circuit 206 explained in FIG. 5, instead of the PWM oscillation circuit 211 in FIG. 8.
Additionally, Japanese Patent Laid-Open No. 8-139382 has proposed a controller for a piezoelectric transformer with a stepped-up output voltage whose waveform is practically regarded as aburst waveform. More practicality, the document is directed cutting the aftershock signal of an output waveform signal of the transformer corresponding to the burst waveform signal. In the document, a method of intermittently operating a half-bridge type driving circuit in accordance with an oscillation signal by using an AND gate circuit has been proposed as a half-bridge type piezoelectric transformer driving circuit related to the control circuit shown in FIG. 2. In this method, however, two transistors connected to form a half-bridge circuit are turned on and off by the same output pulse signal from the AND gate circuit. Accordingly, both the transistors are turned off in the intermittent period of this pulse signal. That is, in the intermittent period of the pulse signal, the driving voltage to a piezoelectric transformer floats from a zero potential to result in an unstable state. Consequently, the output voltage from the piezoelectrictransformer which is supposed to be a zero potential during the period is not necessarily zero. Further, if the cold-cathode fluoresent lamp is driven by the output waveform signal which has been removed of the aftershock (ringing) signal, the life of the cold-cathode fluoresent lamp will be shortened, because the lamp can be damaged by the output waveform signal.